Is the statement about extrinsicRPY in the readme wrong?

[inntoy]

Hi @TixiaoShan, thanks for your execllent work!
However, after reading your answer in #6 and the explaination in the IMU alignment part of readme, I am still puzzled about the physical meaning of extrinsicRPY. In the IMU alignment part you said that "We rotate the attitude measurements by -90 degrees around "lidar-z" axis and get the corresponding roll, pitch, and yaw readings in the lidar frame", which implies that extrinsicRPY is q_l_b. But in the yaml file you gave, the physical meaning of extrinsicRPY is to transform the coordinates in the lidar coordinate system to the imu coordinate system, which is q_b_l.

What's more, in the function imuConverter(), the calculation of q_final also verifies that my statement is correct, q_final is q_w_lk, w is the world frame (usually the ENU coordinate system), lk is the lidar frame in time stamp k. Imu's orientation q_from is q_w_bk, bk is the imu frame in time stamp k. If the meaning of extRPY is according to the readme:
** q_from * extQRPY = q_w_bk * q_lk_bk ≠ q_final**
If the meaning of extRPY is according to my statement:
q_from * extQRPY = q_w_bk * q_bk_lk = q_final
So, is the statement about extrinsicRPY in the readme wrong?

Finally, thanks again for your work. Below is a map created with lio-sam. What a nice and beautiful map!

[TixiaoShan]

@inntoy Thanks for pointing this out. When I wrote the readme and comments, there were some wrong descriptions about rotations. If you feel up to it, please free to send a push to the repo.

It's great to see the results like this.

[inntoy]

@TixiaoShan Thanks for such a quick reply. I'm glad my understanding is correct. I will submit a revised description of extrinsicRPY.

[Richardson-Chong]

@inntoy I also find in ImuConverter function that the q_final expression is wrong. But in my opinion, I preserve the yaml extrinsic rotation and transition which imply the convert a point from imu coordinate to lidar coordinate. And then, I change the q_final expression to Eigen::Quaterniond q_final = q_from * extQRPY.inverse(); and that's it. I infer that the usage of extQRPY and extTrans is right in preintegration and other parts. Is that right?

[inntoy]

@Richardson-Chong I think your approach is correct. extQRPY(extrinsicRPY) is only used to calculate q_final in the code.

[Richardson-Chong]

Have you ever noticed the imu2Lidar and lidar2Imu in imuPreintegration？ In my mind, here the author want to predict states first in imu coordinate and then convert to lidar coor. However, the transition imu2Lidar and lidar2Imu are not right in the expression of
gtsam::Pose3 imu2Lidar = gtsam::Pose3(gtsam::Rot3(1, 0, 0, 0), gtsam::Point3(-extTrans.x(), -extTrans.y(), -extTrans.z())); gtsam::Pose3 lidar2Imu = gtsam::Pose3(gtsam::Rot3(1, 0, 0, 0), gtsam::Point3(extTrans.x(), extTrans.y(), extTrans.z()));.
In my mind they should be:
Eigen::Quaterniond q = Eigen::Quaterniond(extRot); gtsam::Pose3 lidar2Imu = gtsam::Pose3(gtsam::Rot3(q.w(), q.x(), q.y(), q.z()), gtsam::Point3(extTrans.x(), extTrans.y(), extTrans.z())); gtsam::Pose3 imu2Lidar = lidar2Imu.inverse();
And here the IMU msgs will not be transformed by imuConverter because the IMU-Preintergration and IMU-predict work in imu coor. I don't know whether my opinion is right. @TixiaoShan @inntoy

[inntoy]

@Richardson-Chong In imuHandler, with the ImuConverter function, imu raw data (acceleration and angular velocity) has been converted to another coordinate system b*. There is only a translation relationship between the coordinate system b* and the lidar coordinate system.
Let's label the transformation represented imu2Lidar as T_b*_l, and I guesses that the transformation represented by the variable imuPose is T_w_b*. In this situation, lidarPose = imuPose.compose(imu2Lidar); can be expressed as T_w_l = T_w_b* x T_b*_l. So the author is correct

[Richardson-Chong]

You are right. Now the only thing I don't figure out is the gravity acceleration setting of Gtsam.
boost::shared_ptr<gtsam::PreintegrationParams> p = gtsam::PreintegrationParams::MakeSharedU(imuGravity);
Should it be the gravity vector of initial Body coordinate? If it's true, the whole process should be in Body coor, not the changed B* coor.

[inntoy]

@Richardson-Chong In my opinion, the gravity vector should be in the world coordinate (i.e. (0,0,-9.8)). That's why the author said to use a nine-axis IMU, because it can give us the quaternion q_wb.

[Richardson-Chong]

What I mean is that if author use B* as coordinate, should the gravity acceleration also be transformed to B*? But in the code, imuGravity is just equal to g in B.

[inntoy]

@Richardson-Chong Actually, the initial body coordinate (B0) is not necessarily the world coordinate. It can be a rotation between them, which is q_wb0 (get from imu attitude). I think the imu-integration process is carried out in the world coordinate. I also find that initialization in function updateInitialGuess will save the information of q_wb0. But I have not fully understood this part. If you understand this part, please let me know.

[Gatsby23]

不好意思，这里为了理解的更清楚点，就直接中文给您回复了：
关于这里IMU的测量和外参的转换，我还是有点不理解：为什么在imu_handler当中会把IMU的测量值转换到lidar坐标系下，但是在预积分的时候，IMU还需要在自身的坐标系下进行积分呢？我的理解是：整体在状态估计的时候应该是以Lidar作为body系主体，依次向下进行状态递推的

[inntoy]

imuConverter那个函数只是把imu测量值转换到了一个和lidar坐标系只有平移而没有旋转关系的坐标系b*。那预积分其实都是在b*下进行的，那肯定是要加上平移关系才能转换到lidar坐标系下（imu2Lidar里只有平移）。

[Gatsby23]

imuConverter那个函数只是把imu测量值转换到了一个和lidar坐标系只有平移而没有旋转关系的坐标系b*。那预积分其实都是在b*下进行的，那肯定是要加上平移关系才能转换到lidar坐标系下（imu2Lidar里只有平移）。

OK，我大致明白了，我仔细看下源码，后面再和您详细交流下

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[inntoy]

您好，您的邮件我已收到。————祝您每天都有个好心情!

[himhan34]

您好，您的邮件我已收到。————祝您每天都有个好心情!

can i ask you something?

[inntoy]

您好，您的邮件我已收到。————祝您每天都有个好心情!

[himhan34]

Thanks for replying . i am trying to know how to implement the 45 degree tilted velodyne lidar(vlp16) with the imu(3DM-GQ7 GNSS). i just tried to modify extrinsic matrix in the params.yaml.. is there a method that i can do? 2022년 12월 8일 (목) 오후 5:49, Omega_lin ***@***.***>님이 작성:

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[inntoy]

@himhan34 It should be enough to provide extrinsic matrix directly. I guess you are using KAIST dataset? I haven't tested LIO-SAM on this dataset, can you share the running results?

[himhan34]

wow thanks for replying.
i am using my own dataset which is recorded in my lab's corridor .

this is a extrinsic matrix that i used

extrinsicRot: [1,0,0,
0,1,0,
0,0,1]

extrinsicRPY: [0,1,0,
1,0,0,
0,0,-1]

and this is a results.

really thanks for your reply. i am handling this problem in 3 weeks

[inntoy]

wow thanks for replying. i am using my own dataset which is recorded in my lab's corridor .

this is a extrinsic matrix that i used

extrinsicRot: [1,0,0, 0,1,0, 0,0,1]

extrinsicRPY: [0,1,0, 1,0,0, 0,0,-1]

really thanks for your reply. i am handling this problem in 3 weeks

Um, is the coordinate system of each sensor the same as in the picture I drew? att means the attitude, acc means the acceleration. @himhan34

[himhan34]

2022년 12월 8일 (목) 오후 8:59, Omega_lin ***@***.***>님이 작성:

…

wow thanks for replying. i am using my own dataset which is recorded in my lab's corridor . this is a extrinsic matrix that i used extrinsicRot: [1,0,0, 0,1,0, 0,0,1] extrinsicRPY: [0,1,0, 1,0,0, 0,0,-1] really thanks for your reply. i am handling this problem in 3 weeks Um, is the coordinate system of each sensor the same as in the picture I drew? att means the attitude, acc means the acceleration. @himhan34 <https://github.com/himhan34> [image: device] <https://user-images.githubusercontent.com/39400163/206441116-46d1a2a6-cfcf-4eae-ba9b-3e0edfc1e680.png> — Reply to this email directly, view it on GitHub <#335 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ALFL7J6UABWHCYDBPCRMPODWMHES3ANCNFSM5UD6S6PA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[himhan34]

[himhan34]

for the not 45 degree tilted i think the coordinate is like this.

[inntoy]

I found that extrinsicRPY seem to be wrong, it should be the unit matrix, that is:
[1,0,0,
0,1,0,
0,0,1]
@himhan34

[himhan34]

[himhan34]

the pic that i uploaded first was the flat situation,
the real one is the next pic

[himhan34]

interestingly this is a default pointcloud when i play the rosbag in the rviz

[himhan34]

and when i set the parameter like this

and the pointcloud is like this,

[inntoy]

Oh, I see. It looks like your extrinsics are wrong. Maybe you can use a calibration tool to get the extrinsics between LiDAR and IMU. I advise this: https://github.com/hku-mars/LiDAR_IMU_Init. But please note that the extrinsic matrix obtained by using this method is T_imu_lidar. And LIO-SAM uses T_lidar_imu, which can be obtained by taking the inverse of the matrix T_imu_lidar. @himhan34

[himhan34]

and interestingly

Oh, I see. It looks like your extrinsics are wrong. Maybe you can use a calibration tool to get the extrinsics between LiDAR and IMU. I advise this: https://github.com/hku-mars/LiDAR_IMU_Init. But please note that the extrinsic matrix obtained by using this method is T_imu_lidar. And LIO-SAM uses T_lidar_imu, which can be obtained by taking the inverse of the matrix T_imu_lidar. @himhan34

thanks. i will try that, thanks for helping me!.

[inntoy]

@himhan34 Also, according to this picture, I visually estimate that the rotation in the extrinsics should be:
[1, 0, 0,
0,0.707.0.707,
0,-0.707,0.707]

You can write it into your config file, extrinsicRot and extrinsicRPY should be the same.

[himhan34]

extrinsicRot: [1, 0, 0,
0,0.707.0.707,
0,-0.707,0.707]

extrinsicRPY: [1,0,0,
0,0.707,0.707,
0,-0.707,0.707]

i tried but the result was this sadly.

[inntoy]

extrinsicRot: [1, 0, 0, 0,0.707.0.707, 0,-0.707,0.707]

extrinsicRPY: [1,0,0, 0,0.707,0.707, 0,-0.707,0.707]

i tried but the result was this sadly.

Oh, forgive me, looks like the rotation I gave is wrong. Just uses the result from the calibration tool.
Finally, there is another thing you need to pay attention to, your imu needs to output the attitude quaternion.

[himhan34]

extrinsicRot: [1, 0, 0, 0,0.707.0.707, 0,-0.707,0.707]
extrinsicRPY: [1,0,0, 0,0.707,0.707, 0,-0.707,0.707]
i tried but the result was this sadly.

Oh, forgive me, looks like the rotation I gave is wrong. Just uses the result from the calibration tool. Finally, there is another thing you need to pay attention to, your imu needs to output the attitude quaternion.

i will try the calibration tool from the fastlio2 lab. thanks!!
and can you tell me more about the pay attention thing that you told me?
i am not quite familiar to quaternion. and also know that the value that lio_sam publishes either has the orientation
value with quaternion..

[himhan34]

is that a thing that you told me?

[inntoy]

@himhan34 First, check your imu topic name(such as /imu).
Then, print the output of imu in your terminal by this instruction:
rostopic echo /imu
You should see the picture below. The part in the red circle in the picture is the quaternion.

[himhan34]

yes i can see

[inntoy]

is that a thing that you told me?

is that a thing that you told me?

Yes, now you get the extrinsics. It should save the result in somewhere, you can check the README in LiDAR_IMU_init.

[inntoy]

yes i can see

it seems ok.

[himhan34]

thanks i will check it and adept to my config.yaml in lio_sam.
and i have some little question. so i have to multiply the rotation matrix(x,y,z) rotation to ROT,RPY?

[inntoy]

thanks i will check it and adept to my config.yaml in lio_sam. and i have some little question. so i have to multiply the rotation matrix(x,y,z) rotation to ROT,RPY?

thanks i will check it and adept to my config.yaml in lio_sam. and i have some little question. so i have to multiply the rotation matrix(x,y,z) rotation to ROT,RPY?

The saved file will give transform matrix T_imu_lidar. And you should inverse the transform matrix to get T_lidar_imu. Then get the rotation matrix from it. So there is no need to convert Euler angles into rotation matrix.

[himhan34]

This is a thing that you told me!!!!!
then i can inverse this homogeneous transform matrix in matlab and adept to ROT and RPY!

but The Rot and RPY is the same?

[inntoy]

This is a thing that you told me!!!!! then i can inverse this homogeneous transform matrix in matlab and adept to ROT and RPY!

Congratulations! Yes, because your atttitude and acceleration coordinate system are the same, so it should be same. Also, the calibration results are not always correct. If the map you built with LiDAR_IMU_init looks wrong, you need to recalibrate it again.

[himhan34]

thanks!!! have a good day! is it okey that if i have some questions, can i ask you personally in email?

[inntoy]

thanks!!! have a good day! is it okey that if i have some questions, can i ask you personally in email?

Sure, my email is [email protected]

[himhan34]

thanks!! have a good day! 2022년 12월 8일 (목) 오후 11:40, Omega_lin ***@***.***>님이 작성:

…

thanks!!! have a good day! is it okey that if i have some questions, can i ask you personally in email? Sure, my email is ***@***.*** — Reply to this email directly, view it on GitHub <#335 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ALFL7J7BPLVCAWENPVBZARTWMHXONANCNFSM5UD6S6PA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[HerB1998]

@Richardson-Chong Actually, the initial body coordinate (B0) is not necessarily the world coordinate. It can be a rotation between them, which is q_wb0 (get from imu attitude). I think the imu-integration process is carried out in the world coordinate. I also find that initialization in function updateInitialGuess will save the information of q_wb0. But I have not fully understood this part. If you understand this part, please let me know.

I have the same confusion. Why not align the initial body coordinate (B0) with the world coordinate (NEU)? Have you solve the problem?

[inntoy]

@HerB1998 What I said was wrong. First, the imu-integration process is carried out in the local frame b*. Second, the imu's attitude will be q_b0_bn or q_ENU_bn, which b0 is the coordination when the imu starts. The definition of the world coordination w depends on the hardware settings of the imu itself, which is b0 or ENU.

[THARUN-V]

Hi what is the tool used for lidar-imu calibration.

[inntoy]

您好，您的邮件我已收到。————祝您每天都有个好心情!

[Yangchengshuai]

@himhan34 First, check your imu topic name(such as /imu). Then, print the output of imu in your terminal by this instruction: rostopic echo /imu You should see the picture below. The part in the red circle in the picture is the quaternion.

hello，my imu do not ouput attitude quaternion ,The output is all 0.Did it make any difference?

[inntoy]

您好，您的邮件我已收到。————祝您每天都有个好心情!

[himhan34]

@himhan34 First, check your imu topic name(such as /imu). Then, print the output of imu in your terminal by this instruction: rostopic echo /imu You should see the picture below. The part in the red circle in the picture is the quaternion.

hello，my imu do not ouput attitude quaternion ,The output is all 0.Did it make any difference?

sorry for the late reply... you solved that?
if you are okay could you check your manual of the imu and check that, param or setting of quaternion is available?